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I have a LiPo battery pack with 5V, 2.1A output. Now, let's say I boosted that with a step-up regulator like this Pololu module.

At 12V output from the regulator, what is the maximum current I can draw?

By conservation of energy, I'm assuming it cannot be more than 5 V * 2.1 A / 12 V = 0.875 A, but I'm wondering if there are any other factors at play.

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Yes, you can't convert all the power at the input to the output. There are always some energy loss in the conversion.

By your Pololu module's specification, it has a typical efficiency between 80% to 90%. You can refer to their Typical Efficiency and Output Current section to find the maximum current you can get.

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But, this module has input current limit (typically between 1.4 and 2A),

The maximum achievable output current is approximately proportional to the ratio of the input voltage to the output voltage. If the input current exceeds the switch current limit (typically somewhere between 1.4 and 2 A), the output voltage will begin to drop.

So, with 5V input voltage, the input current can't be more then 1.4A. From the graph, we note the efficiency drop as output current increase. At 500mA, the efficiency drop to nearly 80%, then your input current now is

$$ \frac{12V \times 500mA}{0.8 \times 5V} = 1.5A > 1.4A $$

So, your output current can't be more than 500mA, should be about 450mA or so.

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